Modern military aircraft are designed to have as low a radar return or cross-section as possible in an attempt to enable them to evade interception and attack by a radar defence system during a combat operation. It is possible to reduce the radar return or cross-section of an aircraft by modifying its shape, by having the aircraft made from materials which do not reflect radar and by using a material which absorbs electromagnetic radiation to cover those parts which have to be made of a material which reflects electromagnetic radiation.
In spite of the best endeavours which have been made so far all aircraft still provide some radar return and the particular return or radar signature which is obtained from an aircraft is distinctive of aircraft of that particular type. Modern radar systems analyse the radar return which is obtained and use this to give an indication of the type of aircraft. For example, modern radar compares the radar signature of an aircraft at two or more different frequencies and analyses the return for different polarisation states. Phased array radars are particularly discriminating. Typically, at least part of the radar return is sampled to monitor changes that take place as part of its threat assessment. A radar defense system is usually programmed with the radar signatures of that country's own, and other friendly aircraft and also programmed with the radar signatures of those aircraft which are operated by potential enemies. The radar defence system is sometimes controlled so that it will only launch an attack on aircraft which has been identified as having a radar signature corresponding to that of a potential enemy. It is relatively easy for any country to be able to get information on the radar signature of aircraft operated by its potential enemies whilst these aircraft are taking part in training exercises. Thus, at present, it is virtually impossible to keep the radar signature of any aircraft secret and thereby prevent it being recorded and built into a radar defence system of a potential enemy as an enemy aircraft.